The Cell Cycle & DNA Replication
- The cell cycle consists of a period of cell growth and DNA replication called interphase, and then a period of mitosis.
- DNA helicase breaks the hydrogen bonds between the two polynucleotide strands. The helix unzips to form 2 single strands.
- Each original single strand acts as a template. Free DNA nucleotides join to the exposed bases.
- New nucleotides are joined together by DNA polymerase. Hydrogen bonds form between bases on the original and new strand.
- Each new DNA molecule contains one strand from the original DNA molecule, and one new strand.
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Cell Division - Mitosis
- Mitosis has five division stages:
- Cell's DNA is unravelled and replicated to double it's genetic content.
- The organelles are also replicated.
- Chromosomes condense. Tiny protein bundles called centrioles start moving to opposite ends of the cell.
- They form a network of protein fibres called the spindle.
- The nuclear envelope breaks down and the chromosomes lie free in the cytoplasm.
- Chromosomes line up along the middle of the cell and become attached to the spindle by their centromere.
- The centromeres divide, separating each pair of chromatids.
- The spindles contract, pulling the chromatids to the opposite ends.
- Chromatids reach the opposite poles on the spindle and uncoil, forming chromosomes.
- The nuclear envelope forms around each group of chromosomes, so there's two nuclei.
- The cytoplasm divides to form two genetically identical daughter cells.
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Cell Differentiation & Organisation
- Multicellular organisms are specialised. The structure of each specialised cell is adapted.
- The process of becoming specialised is called differentiation.
- Palisade Mesophyll Cells:
- Contain many chloroplasts, so they can absorb lots of sunlight.
- The walls are thin, so CO2 can easily enter.
- Similar cells are organised into tissues, for example:
- Squamous epithelium tissues.
- Phloem Tissue.
- Xylem Tissue.
- Tissues Are Organised Into Organs - e.g. The Leaf:
- Lower Epidermis: Contains stomata for gas exchange, it lets air in and out.
- Spongy Mesophyll: Full of spores to let gas circulate.
- Palisade Mesophyll: Photosynthesis occurs here.
- Xylem: Carries water to the leaf.
- Phloem: Carries sugars away from the leaf.
- Upper Epidermis: Covered in a waterproof waxy cuticle to reduce water loss.
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